There has recently been an increased interest in the improvement of direct-current arc lamps, particulary arc lamps used in combination with optical systems for projection use. When projecting images, it is important that the light generation occur localized highly concentrated and, additionally, that the light which is generated in this localized region be homogeneous. The color separation effect becomes material. The color separation effect can be described as a localized separation, with respect to different spectral regions or colors of the overall light respectively. This localized separation of specific colors within the overall light being generated decreases the quality of light for projection uses, since color boundaries will result at limiting or end or edge regions of projected pictures, slides or images. The color separation effect is generated by the electrical direct-current field which arises during operation of the lamp between the anode and cathode of a direct-current arc lamp. This electric direct-current field influences the distribution of concentration of metal ions which generate the light, between anode and cathode. Thus, the spatial distribution of metal ions between the anode and cathode may become non-homogeneous. Different metal ions may be subject to different distributions of concentration. The respective different metal ions provide different spectral contributions to the overall light output of the lamp and these differences then lead to the undesired color separation effect.
The direct-current arc lamp forming the subject matter of this invention uses a fill containing halogen. During operation of the lamp, metal halides within the discharge vessel will arise. Metal halides have a higher vapor pressure than the corresponding elementary metals. At high arc power, typically about 80 W per millimeter of arc length and more, the light generating metal halides will generate high vapor pressures. This ensures, on the one hand, high light output from the lamp; on the other hand, however, the high vapor pressure enhances, usually, also the color separation effect.
Further criteria for quality of a lamp--not only for projection use--are sufficient proportions of the base colors blue, green and red to ensure good color rendition and a desirable color temperature.